Due to developments in well drilling and engineering technology there is an increasing need for methods that can provide very specific information about the earth formation surrounding the wellbore. The location of a boundary between formation regions having different properties is important for the geologist or the well engineer. It is of particular interest to be able to detect the presence of such a boundary near the wellbore. Such information can be used in order to steer the drilling accordingly. For example, modern drilling technology allows drilling of a horizontal well into a substantially horizontal oil-bearing formation region. An oil-bearing formation can for example be sandwiched between water-bearing formation regions, and in such a case it would be highly desirable to steer the drill bit such that no boundary between the oil-bearing and a water-bearing formation region is crossed.
Other examples in which detection of a boundary between formation regions is desired include the determination of optimum locations to obtain a core sample, or the determination of locations in the wellbore where casing is required.
A boundary in an earth formation generally separates formation regions that differ substantially in at least one characteristic property. In order to detect the boundary by a measurement, the measurement has to be sensitive to the change in the characteristic property. The present invention relates to the situation that the characteristic property is the electrical resistivity, which will in the specification and the claims be simply referred to as resistivity.
Measurements of the resistivity are used in the art for characterizing an earth formation along a wellbore, and such measurements are commonly performed by logging tools, either while or after drilling the wellbore. For example, oil-bearing formation regions generally exhibit a higher resistivity than water-bearing regions. A sudden change in the measured resistivity at a certain location in the wellbore is therefore a typical indication that at this location the wellbore crosses a boundary between formation regions. It is desirable, however, to detect the boundary at a distance in the formation, without the need for the boundary to actually cross the wellbore.
Several types of logging tools for measuring resistivity are known in the art. A particular type are so-called resistivity logging tools which comprise a number of electrodes. These logging tools are operated by emitting electrical currents from one or more electrodes through the wellbore into the formation, and the resulting electrical potential (or an electrical current) is measured at certain electrodes on the logging tool. Determining a resistivity from these measurements is done in a so-called inversion process.
U.S. Pat. No. 3,838,335 discloses a method for determining the presence of and the distance to a horizontal subsurface boundary, using a logging tool in a vertical wellbore. The method comprises emitting a current from a single current electrode on the logging tool, and measuring the potential at two potential electrodes, which are arranged at equal distance (in the order of more than 100 m) above and below the current electrode. The potential electrodes have a length of about one tenth of their distance from the current electrode. A horizontal boundary ahead of the logging tool causes a potential difference between the potential electrodes, which is used to determine the depth of the horizontal boundary.
U.S. Pat. No. 5,038,108 discloses a method for determining from inside a wellbore the distance to a boundary which extends parallel to the wellbore. In the known method, a logging tool having a single emitter electrode is used, and resistivity measurements are performed using a plurality of detector electrodes disposed at increasing distances from the emitter electrode. The results of the measurements are compared with previously obtained reference curves, thereby providing the distance to the boundary.